Coating compositions comprising the adduct of an .alpha.,.beta.-unsaturated dicarboxylic acid to an unsaturated fatty acid, a polyol esterified with an autoxidisable monocarboxylic fatty acid, and an at least two hydroxyl groups-containing monocarboxylic acid, such as dimethylol propionic acid, are known per se from GB-A-1 471 611. However, the binder composition disclosed therein is essentially non-polymeric and moreover comprises a polyol partly esterified with acrylic and/or methacrylic acid. Though the disclosed autoxidisable compounds permit the formulation of quick-drying paints at "very high application solids," due to the presence of acrylyl or methacrylyl groups these paints tend to be unstable under normal storage conditions over typical storage times; that is, they have a problem with what is conventionally known as "pot-life stability."
WO-A-93/17060 discloses the preparation of high-molecular weight alkyd resins starting from polyols, a chain extender such as dimethylol propionic acid, and an unsaturated fatty acid. In this manner dendrimer-type macromolecules are obtained. However, the process of preparing said alkyd resins is cumbersome, as it requires the use of strong acids, such as sulphuric acid and methane sulphonic acid, for proper conversion to dendrimeric material. It should therefore be considered highly surprising that the use of a chain extender as disclosed in WO-A-93/17060 in an alkyd resin composition as disclosed in U.S. Pat. No. 5,053,483 provides alkyd resins which can be used in coating compositions of very low VOC while producing cured films having improved physical characteristics. The invention relates to an alkyd resin comprising an unsaturated fatty acid, a polyol, and the adduct of an .alpha.,.beta.-unsaturated dicarboxylic acid to an unsaturated fatty acid. Alkyd resins of said type have earlier been proposed in U.S. Pat. No. 5,053, 483. The resins disclosed therein comprise a fatty acid drying component, a polyol, a dibasic acid component, and a fatty acid-based trifunctional acid component, wherein a portion of the trifunctional acid component is a 22-carbon aliphatic acid anhydride which is a Diels-Alder adduct of maleic anhydride to linoleic or linolenic acid.
Though the known alkyd resins have a significantly reduced viscosity, allowing for formulations with reduced amounts of solvents, the performance of the cured films produced therefrom is poorer in terms of durability, such as cracking and gloss retention. Moreover, there is a constant need in the art for non-aqueous coating compositions having a reduced volatile organic content (VOC).
It is an object of the present invention to provide an alkyd resin which makes it possible to provide coating compositions of very low VOC while producing cured films having improved physical characteristics.